Heat-treatment of electroplated strip metal



March 25 194* J. s. NACHE'MAN 2,418,033

EBAY? TREATMENT OF ELECTROPLATEQ STRIP METAL Filed March 30, 1942 arwwWO Patente d Mar. 25, 1947 HEAT-TREATMENT OF ELECTROPLATED STRIP METAL John S. Nachtman, Youngstown, Ohio Application March 30, 1942, Serial No. 436,783

12 Claims.

The invention relates generally to the heat treatment of electroplated strip metal, and more particularly to the heat treatment of strip steel electroplated with metals or alloys having low melting points, such as tin, terne and the like, in order to brighten and improve the coating.

Prior practice includes heat treating metal in a non-oxidizing or reducing atmosphere such as hydrogen or carbon monoxide, but where strip metal is heat treated continuously the use of such atmospheres is very expensive and requires excessively long time periods or long heat treating chambers or both. Moreover, such atmospheres are not well adapted for heat treating electroplated coatings such a tin, because of the rapid temperature changes required in the coated strip.

In order to brighten and improve the quality of the matte-dike surface which i characteristic of electroplated tin coatings, in a minimum amount of space, I have determined that it is desirable first to heat the coated strip rapldli and uniformly in a non-oxidizing hydrocarbon medium to a temperature just below the melting point of 449.5" E. of tin. Then the temperature of the coated strip is raised quiclrly to slightly above the melting point of tin for a short time to make the tin coating molten and close the pores thereof. While still in a non-oxidizing medium and Without physically contacting any thing else, the coated strip is then rapidly cooled to a temperature which is below the point of solidification of tin or below about 435 F., and then the coated strip is cooled further to below the temperature at which tin will oxidize, before being exposed to the atmosphere.

It is therefore an object of the present invention to provide novel methods and apparatus for brightening electroplated coatings.

Another object is to provide novel methods and apparatus for heat treating coated strip continuously.

A further object is to provide a novel method for rapidly heating and cooling. continuous electroplated strip to predetermined temperatures.

Another object is to provide a novel and compact apparatus for heat treating strip electroplated with low melting point. metal so as to brighten the electroplated surface.

A still further object is to provide a novel atmosphere for continuously heating and cooling electroplated strip.

These and other objects are accomplished by carrying out the heat treatment of the electroplated strip in a non-oxidizing medium of one or more hydrocarbons and having a boiling point below the melting point of tin, controlling the temperature of aid medium to render the same liquid and gaseous at various stages, to rapidly and uniformly heat the strip, to momentarily melt the coating thereon, and then to rapidly cool the strip to a temperature below that at which the coating will oxidize.

Referring to the drawing forming part hereof, the figure is a diagrammatic cross sectional view of a preferred embodiment of apparatus for carrying out the invention.

The strip 8 prior to entering the improved apparatus preferably has been electroplated with tin, and rinsed and dried according to conventional practice. The strip is then passed over a roll 9 and enters the apparatus through a seal indicated at it. which may be a soft material such as synthetic rubber or asbestos which presses slightly against the surfaces of the strip.

From the seal t0, the strip passes downward into a compartment ll filled with a medium indicated at which. is preferably a saturated or unsaturated aliphatic hydrocarbon or an arcmatic hydrocarbon or a mixture of two or more which boils below the melting point of the tin coating on the strip, preferably between 300 F. and 4l0 E, and which does not deposit carbon film or smudge when in contact with molten tin. Saturated and unsaturated aliphatic hydrocar bons and mixtures thereof include alkanes hav-- ing carbon chain of three or more atoms, olefins, and petroleum fractions or solvents. Aromatic hydrocarbons include coal tar fractions or solvents.

Petroleum and coal tar fraction in this boiling range are light hydrocarbons as distinguished from heavy oils, and have high rates of thermoconductivity as compared with heavy oils, but do not oxidize or vaporize to any substantial extent at room temperature. They have relatively high thermostability and do not decompose or crack when in contact with molten tin so as to leave a carbon smudge thereon.

Medium A may be a petroleum fraction or sol" vent known as Varsol. Varsol is a petroleum fraction having a boiling range of about 310 F. to 395 F., a flash point of approximately F., a vapor pressure of only 3 mm. of mercury at 77 F., and a viscosity of .92 centipoise at 77 F. Varsol can be heated to the soldering temperature of tin without causing any carbon smudge.

In compartment H the medium A is kept in a liquid state with a liquid level L by means of a water jacket l2 under the seal l0 and cooling coils I3 on opposite sides or the downwardly moving strand of the strip which then passes around and under a roll 14 in the lower part of compartment II. This may be termed the first stage of treatment.

A partition wall l located above roll I4 divides the upper part of compartment H into two parts so that the level L of the liquid A to the left of the partition wall is kept cool by the water jacket l2 and cooling coils l3. From the roll ll the strip moves upwardly into a preheating compartment l6 where the strip is given the second stage of treatment, and compartment 16 communicates with compartment H on the right side of the partition wall l5.

Electric heating elements I! or other heating means are located on opposite sides of the strip in the preheating compartment l6 and extend downwardly therefrom into the medium A on the right side of partition wall l5. In the top portion of the preheating compartment the strip passes over a roll I8 and then downwardly into a fusing compartment l9 where additional heat is supplied by heating elements 20, and where the third stage of treatment takes place. The strip then passes through a hole or slot in partition wall 2| into a cooling compartment 22, where the fourth stage of treatment takes place.

The heating elements I! vaporize the medium A at its liquid level L on the right side of the partition wall l5, with the result that the hydrocarbon vapor fills the preheating, fusing and cooling compartments l6, l9 and 22 and builds up a pressure therein. This pressure lowers the liquid level L' and raises the liquid level L until a. sufficient amount of the heating elements I! is uncovered to establish an equilibrium between the pressure in the compartments [6, l9 and 22 and the rate of heating and vaporization at the liquid level L.

As the strip enters the preheating compartment iii, the temperature of the strip is somewhat below the 400 F. temperature of medium A at level L and as the strip passes through compartment i6, its temperature is raised rapidly and uniformly in the following manner.

The vapor in compartment 16 is superheated to a temperature materially above its temperature of vaporization by the heating elements IT. The temperature difference between the temperature of vapor in compartment l6 and the temperature of the strip as it leaves medium A at level L is even greater. Therefore, when the vapor contacts the strip in compartment IE it condenses on the strip whereby the heat of vaporization and of superheating the vapor is given up to the relatively cooler strip very rapidly so as to rapidly and uniformly heat the strip. As the strip continues to pass upward in compartment l6 its temperature is further raised by the heating element l1 and superheated vapors, quickly and uniformly to just below 449.5" F.

After the strip passes over the roll l8 into the fusing compartment 19, the additional heat supplied by the heating elements 29 raises the temperature of the strip sufficiently to melt and flow the coating on the strip, thus brightening the tin coating on the strip. Accordingly, medium A must be a hydrocarbon which will not leave a smudge or deposit on the strip.

Circulation fans 23 are provided in the preheating, fusing and cooling compartments I6, I9 and 22 to keep the hydrocarbon vapor circulating and thus speed up the heating rate and improve the uniformity of the strip. The amount of heat supplied by the heating elements I! in the preheating chamber is controlled to raise the temperature of the strip to about 440 F. or Just below the melting point of the tin coating by the time the strip passes over the top roll II. The amount of heat supplied by the heating elements 29 is controlled so that during the short time (about 1 to 30 seconds) the strip is in the fusing compartment IS the coating is raised to slightly above the melting point of tin, or approximately 450 F. to 470 F.

From the fusing compartment IS the strip passes through a slot in the partition wall 2| into the cooling compartment 22 in which the heat of the strip is rapidly absorbed by the cooling coils 22a located on opposite sides of the strip. The strip is then sprayed by spray pipes 24 with liquid hydrocarbon supplied by pipe 25 from a distilling return pipe from the primary cooling compartment as will be later described. Since the strip surface will be at'a temperature above the boiling point of the hydrocarbon, a rapid vaporization of the sprayed hydrocarbon will take place on striking the strip and the heat of vaporization will be taken from the strip and cause a rapid cooling of the same.

The strip passes downwardly from the cooling compartment 22 into a primary liquid cooling compartment 26, around and under a roll 21, then up and over a roll 28 and again downward into a secondary liquid cooling compartment 29. The compartment 26 is preferably filled with the same hydrocarbon as medium A at the start of the operation, and the said hydrocarbon is kept liquid by means of cooling coils 30. However, compartment 26 could be filled with a different hydrocarbon having a diiferent boiling point between 300 F. and 440 F. The secondary liquid cooling compartment 29 is preferably filled at the start of the operation with a hydrocarbon of lower boiling point, as for example pentane having a boiling point of about 97 R, which acts as a solvent for the carry over of hydrocarbon medium A on the strip, and which evaporates quickly from the strip surface as it leaves compartment 29 at or near room temperature.

The liquid hydrocarbons in compartments 26 and 29 can be maintained at desired temperatures by regulating the cooling solution flowing through the cooling coils 30 and 3| to insure that the temperature of the strip leaving compartment 29 is below the point at which the tin coating will oxidize, or approximately at room temperature.

On leaving compartment 29 the strip passes through the water jacket 32 and a seal 33 in order to minimize losses of the solution to the atmosphere, and then passes over a discharge roll 34.

A small percentage of vegetable oil such as palm oil or cottonseed oil may be added to the liquid pentane or other low boiling hydrocarbon in compartment 29 to provide a very thin film of oil on the strip surface as it emerges from the apparatus, and in order to insure an even distribution of the oil film and prevent streaking. the strip passes between a pair of squeegee rolls 35 preferably provided between the liquid level in compartment 29 and the water jacket 32. As the strip emerges from compartment 29 the pentane will quickly evaporate leaving the thin oil film on the strip.

If it is desired, an oil such as palm oil may be added to the liquid hydrocarbon A in compartmerit II as long as the boiling point of the mixture is not raised above about 440, which is just below the melting point of tin. This oil will adhere to the strip to form a film thereon as it passes through the preheating and fusing compartments.

In order to separate out any oil carried over on the strip into the liquid solution in compartment 26, or to separate out medium A where a different hydrocarbon is used in compartment 26, the solution is drawn off from that compartment through pipe 36 and pumped by means of a pump 31 through a distilling apparatus indicated at 38. The hydrocarbon corresponding to that contained in compartment 26 is returned to compartment 26 through pipe 25 and sprays 24. The oil and hydrocarbon medium A is returned to compartment ll through the oil return pipe 40.

A similar distilling apparatus 4| may be provided for separating the hydrocarbon carried over'on the strip from compartment 26 into the hydrocarbon in compartment 29. Liquid is drawn off from compartment 29 through pipe 42 to apparatus 4|, and the liquid corresponding to that in compartment 26 is returned thereto through pipe 43, while the liquid corresponding to that in compartment 29 is returned thereto through pipe 44.

Thus the hydrocarbon medium A and the liquid hydrocarbon in compartment 26 may be the same hydrocarbon, such as an alkane Or a petroleum solvent, having a boiling point or boiling range below that of the electroplated coating on the strip. Within the scope of the invention, the liquid hydrocarbon in compartment 26 may be a mixture of diiferent hydrocarbons than in medium A so as to boil below 449.5" E, or an entirely different aliphatic or aromatic hydrocarbon or mixture of such hydrocarbons having a boiling point or boiling range below the melting point of tin.

If desired, the hydrocarbon medium in any of the compartments may be introduced therein by means of sprays or jets directed against the strip.

The improved method and apparatus provides for continuously brightening an electroplated strip in a minimum of space because the hydrocarbon atmosphere utilized is such as to effect a rapid heat transfer to and from the strip, so that the strip is heated and cooled rapidly and uniformly. Also, the hydrocarbon atmosphere is sealed at the entrance end by the liquefied medium A and at the exit end by the liquid hydrocarbons in compartments 26 and 29 so that there can be no oxidation of the coating metal during treatment.

Moreover, the coating is given a bright surface of extremely high quality due to the fact that the coatin is molten for a short period of time and is prevented from oxidizing. The improved method and apparatus provides for easy regulation of temperatures over a wide variation at the various stages throughout the method, so hat a variety of coatings of different metals of low melting points or alloys thereof can be bright ened.

The term hydrocarbon medium as used herein means a saturated or unsaturated aliphatic hydrocarbon or an aromatic hydrocarbon, or a mixture of two or more of said hydrocarbons, which boils below the melting point of the coating on the strip and preferably between 300 F. and 440 F., and which does not deposit carbon film or smudge when in contact with the molten coating.

While I have described the heat treatment of electroplated strip metal, the invention may be electro plated metal and preheat the same, then momentarily'melting the tin coating in a second portion of said vapor maintained at a temperature above th melting point of tin, then quickly cooling the electroplated metal to below the point of solidification of said tin coating by spraying a portion of the hydrocarbon medium thereon; and then passing said electroplated metal directly through a liquid hydrocarbon maintained at a temperature which will cool the tin coating below its oxidation temperature.

2. The method of heat treating-metal electroplated with tin, which includes heating a hydrocarbon medium selected from the group of petroleum and coal tar fractions which boil below the melting point of tin to vaporize the same and confining said vapor; passing said electroplated metal directly through said vapor in successive stages maintained at different temperatures to condense some of said vapor on the electroplated metal and preheat the same, to then momentarily melt the tin coating thereon, and to then quickly cool the same to below the point of solidification of said tin coating; and then passing said electroplated metal directly through a liquid hydrocarbon maintained at a temperature which will cool the tin below its oxidation tem perature.

3. The method of heat treating metal electroplated with tin, which includes heating a hydro carbon selected from the group of petroleum and coal tar fraction which boil below the melting point of tin to vaporize said hydrocarbon and confining said vapor; passing said electroplated metal directly through a portion of said confined vapor to condense some of said vapor on the electroplated metal and preheat the same, then momentarily melting the coating in a second portion of said vapor maintained at a temperature above the melting point of tin, and then quenching the tin coating in a liquid third portion of said hydrocarbon and vaporizing some of said liquid hydrocarbon on the tin coating to quickly cool the same.

4. The method of heat treating metal electroplated with tin, which includes momentarily melting the tin coating in the vapor of a hydrocarbon selected from the group of petroleum and coal tar fractions which boil below the melting point of tin, and then quenching the tin coating in a liquid portion of said hydrocarbon and vaporizing some of said liquid hydrocarbon on the tin coating to quickly cool the same.

5. The method of continuously heat treating metal strip electroplated with tin, which includes passing the strip through a liquid hydrocarbon medium selected from the group of petroleum and coal tar fractions which boil below the melting point of tin, heating a portion of said liquid hydrocarbon to vaporize the same and confining said vapor in a closed space commmunicating with the liquid level of said liquid hydrocarbon;

passing said strip directly through a portion of said confined vapor to condense some 01' said vapor on said strip and preheat the strip, then passing the strip through a superheated portion of said vapor to momentarily fuse the coating thereon, and then passing the strip through a portion f aid vapor maintained at a temperature which will solidify the tin coating.

6. The method of heat treating material electroplated with tin to brighten the tin, which includes passing the material through a hydrocarbon medium comprising a petroleum fraction which boils between 300 F. and 440 F., said hydrocarbon being confined and having varying temperature at successive stages, maintaining the temperature of the hydrocarbon at one stage to vaporize it, and preheating the material in said stage by condensing some of said vaporized hydrocarbon on said material, maintaining the temperature of the vaporized hydrocarbon above the melting point of tin in the next successive stage to fuse the coating, and maintaining the temperature of the vaporized hydrocarbon below the melting point of tin in the next successive stage to cool the material below the solidification point of said coating.

'7. The method of heat treating material electroplated with tin to brighten the tin, which includes passing the material through a hydrocarbon medium selected from the group of petroleum and coal tar fractions which boil between 300 F. and 440 F., said hydrocarbon medium being maintained at varying temperatures at successive stages, maintaining the temperature of the hydrocarbon below its boiling range in the first stage to maintain said hydrocarbon in a liquid state, vaporizing the hydrocarbon in the next stage and condensin some of it on the material to preheat the material maintaining the temperature of the vaporized hydrocarbon above the melting point of tin in the next successive stages to fuse said coating, and maintaining the temperature of the vaporized hydrocarbon below the melting point of tin in the next successive stage to cool the material to below the solidification point of said tin coating.

8. The method of heat treating material electroplated with tin to brighten the tin, which includes passing the material through a hydrocarbon medium comprising a coal tar fraction which boils between 300 F. and 440 F., and maintaining the hydrocarbon at varying temperatures at successive stages, maintaining the temperature of the hydrocarbon medium in the first stage to vaporize said hydrocarbon and preheat the material by condensing some of the medium on said material, maintaining the temperature of said vaporized hydrocarbon in the next successive stage above the melting point of tin to fuse said coating, maintaining the temperature of said vaporized hydrocarbon in the next successive stage below the melting point of tin to cool the material below the solidification point of said coating and maintaining the temperature of said hydrocarbon in the next successive stage in a liquid state and below the temperature at which the tin coating will oxidize to further cool said material.

9. The method of heat treating material electroplated with tin, which includes heating a hydrocarbon medium selected from the group of petroleum and coal tar fractions which boll between about 300 F. to 440 F. to vaporize the same and confining said vapor, passing said material through said heated confined vapor to condense some of said vapor on said material and preheat the material, passing said material through a superheated portion of said vapor to momentarily melt the tin coating thereon, and then spraying the material with liquid hydrocarbon maintained at a temperature below the melting point of tin to cool the material quickly below the point of solidification of said coating.

10. In a method of heat treating metal electroplated with tin, the steps of vaporizing a hydrocarbon medium selected from the group of petroleum and coal tar fractions which boil between about 300 F. and 440 F., and then passing the tin coated metal through said vaporized hydrocarbon to condense some of the vapor on the surface of the coated metal for heating the coated metal to a temperature below the melting point of the tin coating.

11. In a method of continuously heat treating a continuous metallic strip electroplated with tin, the steps oi passing the coated strip through a heated vaporized hydrocarbon selected from the group of petroleum and coal tar fractions which boil at a temperature below the melting temperature of tin to condensesome of said hydrocarbon vapor on the electroplated coating and raise its temperature to a value below the melting point of tin, then without exposing the strip to the atmosphere momentarily raising the strip temperature above the melting point of tin, and then quenching and cooling the strip to below the oxidizing temperature of tin before exposing the strip to the air.

12. The method as defined in claim 11 wherein the quenching and cooling of the strip to below the oxidizing temperature of tin is carried out in a hydrocarbon medium selected from the group of petroleum and coal tar fractions which boil below the melting temperature of tin.

JOHN S. NACHTMAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS British Dec. 4, 1936 

